Can one use a topical vitamin c serum while on Accutane? Will it help fade red marks?

I would definitely ask your board certified dermatologist if they recommend this for you during your treatment.  Every patient is different.  However, I tend to recommend only non-irritating, bland products for sensitive skin during isotretinoin therapy.  I don't recommend using anything that can potentially irritate the skin and have found that vitamin C can be a common culprit causing skin sensitivity.  Again, my recommendation is to use unmedicated, sensitive skin products during isotretinoin therapy.